Multi-beam scanner with mechanically moving element for scanning imaging surfaces

ABSTRACT

A multibeam scanning system for scanning an imaging surface, includes at least one radiation emitter configured to emit a first beam of radiation and a second beam of radiation. A spin deflector, rotatable about a spin axis, is provided to direct the first beam to form a first scan line and the second beam to form a second scan line on the imaging surface. At least one moving element, such as a translating lens, disposed upstream of said spin deflector, operates to deflect at least one of the beams with respect to the spin axis of the spin deflector.

RELATED APPLICATION

This application is related to application Ser. No. 08/687,931, entitled"BEAM ERROR CORRECTION USING MOVABLE CORRECTION ELEMENT", filed Jul. 29,1996 and to application Ser. No. 08/687,928, entitled "BEAM ALIGNMENTCORRECTION ELEMENT ASSEMBLY", filed on Jul. 29, 1996.

1. Technical Field

The present invention relates to beam scanning systems and moreparticularly to multi-beam scanning systems with a mechanically movingelement for deflecting a beam in one or two directions to scan imagingsurfaces such as those utilized in offset printing operations.

2. Background Art

Modern electronic prepress operations utilize laser scanning systems towrite or record images for subsequent reproduction or to scan aprerecorded image at a predefined resolution rate. Such scanning systemsmay write or record images or scan prerecorded images on variousprepress media including, photo or thermal sensitive paper or polymerfilms, photo or thermal sensitive coatings or erasable imaging materialsmounted onto an image recording surface or photo or thermal sensitivepaper, polymer film or aluminum base printing plate materials, all usedin electronic image reproduction. Such media are mounted onto arecording surface which may be planar but which is more typically curvedand scanned with a recording or scanning beam.

The primary components of such a system include a recording surface,usually a drum cylinder and a scan mechanism disposed and movable withinthe drum cylinder. The system also includes a processor, with anassociated storage device, for controlling the scanning mechanism andfor scanning a prerecorded image, a photodetector and detectorprocessor. The processor and associated storage device may be housedwithin the system itself or separate from the system with appropriateinterconnection to the system.

The processor, in accordance with stored programming instructions,controls the scanning mechanism to write or read images on the plate orother medium mounted to the inner drum cylinder wall by scanning one ormore optical beams over the inside circumference of the drum cylinderwhile the drum cylinder itself remains fixed.

The scanning and hence the recording are performed over only a portionof the cylinder inner circumference, typically between 120° and 320° ofthe circumference of the drum cylinder. The optical beam(s) aretypically emitted so as to be parallel with a central axis of thecylinder and are deflected, by for example, a spinning mirror, Hologonor Penta-prism deflector so as to form a single scan line or multiplescan lines which simultaneously impinge upon the recording surface. Thedeflector is spun or rotated by a motor about an axis of rotationsubstantially coincident with the central axis of the drum cylinder. Toincrease the recording speed, the speed of rotation of the beamdeflecting device can be increased. To even further increase therecording speed, multiple beam scanning has been previously proposed.

One such proposed multiple beam scanner has utilized a spinning doveprism with a single light source, as discussed, for example, in U.S.Pat. No. 5,214,528. Using a dove prism beneficially allows the use of amultiple beam source, e.g. a laser diode array, while eliminating theneed for multiple beam correction elements and associated hardware.Additionally, for reasons which need not be discussed here, the scanspeed of multiple beam systems using a dove prism can exceed that ofother types of proposed multi-beam systems.

U.S. Pat. No. 5,097,351 proposes another type of multi-beam scanningsystem which utilizes a controlled piezo-reflecting mirror, or whatmight be better characterized as a wobbling mirror, in lieu of a doveprism. In this system, each of two laser beams follow separate opticalpaths. Each optical path has focusing and collimating lenses and anacousto-optical modulator (AOM). Hence, the proposed system requiresboth AOMs and a wobbling mirror. The controlled wobble reflector isdisposed in only one of the optical paths, i.e. the wobbling mirrorreflects only one of the two beams, and is driven to redirect, e.g.rotate, the reflected beam in synchrony with the rotation of the spindeflector. Errors in the direction of the redirected beam are detected,and corrected by driving the wobbling mirror to adjust angular alignmentduring recording operations.

It is of primary importance that the multiple light beams contact thespin deflector as close as possible to a desired location to ensure thatthe appropriate scan lines are formed on the recording surface and hencethe desired image is properly recorded. This includes maintaining thedesired spacing or overlapping relationship of the simultaneouslyscanned beams with respect to each other and the reduction orelimination of any differential scan line bow between successive scanlines.

A wobble, for example in the spinning dove prism of the '528 patent ordegradation in performance due to wear in moving components of thesupport structure, will cause a misalignment of the element with respectto the spin deflector and can create significant banding artifacts inthe scan lines which repeat every two scan passes. The effect ofmisalignment on system banding can be reduced, for example, byincreasing the ratio of the beam diameter at the spinning prism to thebeam diameter at the spin reflector. However, alignment errors, can alsocreate a twinning between groups of the multiple beams. This is becausein the proposed systems the spin prism rotates only half a turn for eachfull turn of the spin deflector. Accordingly, if a misalignment exists,the multiple beam system is restricted to recording during only everyother rotation of the spin deflector to obtain high quality results. Afour beam system is accordingly only two times faster than single beamsystem, an eight beam system only four times faster, and so on.

Further, in multi-beam systems at least one of the light beams must beredirected in synchrony with the rotation of the spin deflector. Anysynchronization errors between the redirection of the redirected beam(s)and the angular position of the spin deflector will make it impossibleto obtain a proper scan of the recording surface and result in improperor unsatisfactory recording of the image. Small changes in the phaselocking of spin or wobble element motion and spin deflector rotation cancreate banding groups.

If the multiple beams leaving the dove prism described in the '528patent or the beam reflected from the wobbling element described in the'351 patent drift or flutter relative to the spin deflector, due forexample to small motor speed variations or wobble mirror driverinaccuracies, the cross-scan spacing between the multiple scan lines maychange, one or more scan lines may bow and/or scan lines may becomenon-parallel even to the point of intersecting, and this will createvisible artifacts, e.g. banding. The '351 patent, as can best beunderstood, proposes a technique for correcting synchronization errorsbetween the described wobbling mirror and the spin deflector using aquad detector and feedback arrangement to control the wobbling mirror toadjust the motion of the wobbling mirror to correct for synchronizationerrors.

The above reference related Ser. No. 08/687,931 application discloses animproved multi-beam scanning system which includes a spinning element,such as a dove or wedge prism in conjunction with a correction element,such as described in the above referenced related Ser. No. 08/687,928application, to correct beam alignment errors in a spin element of thetype described in the '528 patent. The above referenced Ser. No.08/687,931 application also discloses improved techniques for correctingsynchronization errors between the rotation of the spin or wobbleelement and the spin deflector. Accordingly, the inventions described inthe above referenced related applications, which are incorporated hereinin their entirety by reference, can be utilized to improve on previouslyproposed systems, such as those disclosed in the '528 patent, bycorrecting misalignments in the spin element, and hence the misalignmentof the beam being rotated by the spin element with respect to the spindeflector. Additionally provided are improved techniques for correctingsynchronization errors between the spin or wobble element and the spindeflector of the '528 or '351 patents. However, the system described inthe Ser. No. 08/687,931 application requires a spin element as well as acorrection element such as a translating lens.

OBJECTIVE OF THE INVENTION

It is an object of the present invention to provide a multi-beamscanning system which does not require a spin or wobble element torotate one or more of the multiple beams.

It is another object of the present invention to provide a multi-beamscanning system with reduced banding and/or twinning.

It is a further object of the present invention to provide a multi-beamscanning system which scans multiple beams having a desired geometricrelationship with respect to each other.

It is yet another object of the present invention to provide amulti-beam scanning system which scans multiple beams in a desiredmanner with respect to the spin axis of a spin deflector.

Additional objects, advantages, novel features of the present inventionwill become apparent to those skilled in the art from this disclosure,including the following detailed description, as well as by practice ofthe invention. While the invention is described below with reference topreferred embodiments for electronic prepress applications, it should beunderstood that the invention is not limited thereto. Those of ordinaryskill in the art having access to the teachings herein will recognizeadditional applications, modifications, and embodiments in other fields,which are within the scope of the invention as disclosed and claimedherein and with respect to which the invention could be of significantutility.

SUMMARY OF THE INVENTION

In accordance with the present invention, a multibeam scanning systemand method are provided which are particularly suitable forimplementation in high quality graphic arts image setters, platemakersor scanners. The scanning system includes one or more laser light orother type of radiation emitter or emitter array which emits one or morebeams on a path(s) directed towards a spin deflector, such as a spinmirror, Hologon or Penta-prism, configured to deflect and scan the beamsonto a curved imaging surface of, for example a cylindrical drum torecord or write the image.

In accordance with the present invention, the multi-beam scanning systemincludes at least one radiation emitter configured to emit a first beamof radiation and a second beam of radiation. A spin deflector, rotatableabout a spin axis and configured to direct the first beam to form afirst scan line and the second beam to form a second scan line on theimaging surface is also provided.

A beam deflecting translating lens element or an acousto-optic modulatorelement (AOM) is disposed in the path of one or more of the beams,upstream of the spin deflector. Each AOM is operable to deflect a beamwith respect to the spin axis of the spin deflector and in synchronywith rotation of the spin deflector. Each AOM can serve to both modulateand deflect a beam simultaneously or may be used only to deflect a beam.The translating lens element may be translatable in two directions,which are preferably substantially orthogonal to each other. Thetranslating lens element can thus simultaneously deflect a beam in thetwo directions. The deflection of the beam in the first and seconddirections causes the beam to be redirected so as to move inrelationship with the spinning of the spin deflector about the spinaxis, such that the first scan line and the second scan line arenon-intersecting, preferably parallel and have minimal bow.

A pixel clock, and beneficially a clock phase shifter, may be providedfor timing the emitting of radiation which forms the deflected beam orfor modulation of the beam by the AOM element. In such a case, theacousto-optic modulator or translating lens element is operated todeflect the beam so as to move linearly, i.e. form a single unfocusedline scan, across the spin deflector and the pixel clock and clock phaseshifter are operated in synchrony with the operation of the applicableelement to phase shift the beam. Hence if a pixel clock is utilized, thetranslating lens need only be translatable in a single direction. Thepixel clock may control emission from a radiation source, such as alaser diode, rather than the AOM, if desired. The deflection of the beamcauses the deflected beam to be redirected and move linearly across thespin deflector by an amount equal to one pixel in length at the imagingsurface and in relationship with its rotation about the spin axis, suchthat the first scan line and the second scan line are non-intersectingand preferably parallel with minimal bow. It should be understood thatthe scan length across the spin mirror is relatively short as comparedto the unfocused diameter of the beam at the spin mirror. The beamsimpinging upon the spin mirror are overlapping. The amount of overlapwill depend on the desired spot size, optical gain and other factorswhich will be recognized by those skilled in the art. The phase shiftingof the beam causes the first and second scan lines to be in phase.

Using a pixel clock, auxiliary deflection of a laser beam about a twodimensional image plane is performed without requiring physicaldeflection of the laser beam in two orthogonal directions. Hence, theequivalent of two dimensional deflection is achieved with only a singleaxis deflector by applying timing variations to the writing beam pixelclock in synchronism with the motions of a single axis auxiliarydeflector, e.g., an AOM, or moving element such as a translating lens oroscillating mirror. In this way, beam displacement errors occurring inany on-axis beam deflection system such as those used in an internaldrum recorder can be compensated. Such compensation can be used forcorrection of displacement errors, such as bow, etc., produced by othercomponents in a scanning system, particularly in the applicationsrequiring multiple writing beams.

It will be understood that the pixel control uniquely takes advantage ofthe fact that the spinner rotates the axes of the writing head. Sinceany single axis in the writing head projected onto the imaging surfaceultimately has components in both orthogonal axes of the writing surfaceas the spinner rotates, the fixed axis of the image representing time,i.e., the in-scan axis of the image surface, can be used to reduce adeflection along a given axis of the writing head to a single cross-scandeflection component orthogonal to the time or in-scan axis on the imagesurface. Therefore, two direction, i.e., 2D deflection at the imagesurface is provided by proper phase control of the clock signal and asingle axis deflection at the writing head. Using an AOM to create thedeflection at the writing head results in total electronic control ofthe beam in two dimensions at the image with no moving parts and withresponse times of less than a microsecond possible.

In contrast to the clocked single axis deflection which sweeps the beamlinearly back and forth across the surface of the spinner, using theearlier described two dimensional deflection at the writing head causesthe beam to track the rotation of the spinner and actually move, e.g.rotate, with it so as to keep the beam on a fixed point on the spinnersurface. The latter requires a nonlinear sinusoidal correction functionwhile the former requires a tan (angle/2) function which is nearlylinear and can be approximated to within 5% error by a simple linearsawtooth function. It will be recognized that linear ramp functions maybe more beneficial for AOM deflection because the traveling acousticgrating produced in the AOM has a fixed relationship throughout thelength of the ramp function thereby avoiding the time varying distortionof the beam shape associated with diffraction by a non-linear grating.

In accordance with other aspects of the invention, an acousto-opticmodulator element and a translating lens may both be included in thesystem. These elements are conjunctively operated in synchrony with eachother to deflect a beam in two directions. For example the acousto-opticmodulator may deflect the beam in a first direction, while thetranslating lens element is translated to deflect the beam in a seconddirection, which is preferably substantially orthogonal to the firstdirection. The deflection of the beam in the first and the seconddirections causes the beam to be redirected so as to move, e.g. rotate,in synchrony with the rotation of the spin deflector about the spin axissuch that the first and second scan lines are non-intersecting andpreferably parallel with minimal bow.

In accordance with still other aspects of the invention, the scanningsystem may include a detector, such as a quadrature detector, configuredto detect the geometric relationship between the deflected beam(s) andanother beam or a reference corresponding to the spin axis of the spindeflector, after deflection of the beam(s) with respect to the spin axisof the spin detector. The detector may include a photosensor, e.g.charge couple device (CCD), a photodetector, e.g. a photodiode array, orany other suitable light detection device. As may be desirable, thedetector can be configured to detect the geometric relationship prior toand/or during writing on the imaging surface. A controller is typicallyincluded to control the operations of the translating lens and/oracousto-optic modulator element, as well as the pixel clock and clockphase shifter, in accordance with the detected relationship.

In operation, the radiation emitter(s) could be configured to transmitthe first beam along a path so as to impinge upon the spin deflector ata first location relative to the spin axis of the spin deflector and thetranslating lens and/or the acousto-optic element could be configured todeflect the second beam onto a path so as to impinge upon the spindeflector at a second location which is different than the firstlocation. In such a case, the first location will ideally be coincidentwith the axis of rotation of the spin deflector which preferablycoincides with the longitudinal axis of a cylindrical drum on which theimaging surface is mounted.

However, it is more likely that even if the first beam is intended toimpinge upon the spin deflector at its spin axis, some degree of errorwill exist such that an offset will exist between the point of contactof the beam on the spin deflector and the spin axis. Therefore, it mayin many, if not most, instances be preferable to deflect all beams so asto impinge the mirror at desired locations which are offset from thespin axis. For example, it may be advantageous for multiple beams toimpinge upon the spin deflector at an equal distance from the spin axisor offset from the spin axis and equally spaced from each other. Itwould appear advantageous for the respective beams to impinge on thespin deflector relatively close to the spin axis and, to the extentpracticable, in opposed paired locations. In such a case a separatedeflection element (i.e. translating lens or beam deflecting AOM) isdisposed in each beam path, although this may not necessarily bemandatory.

From the above, it can be seen that in accordance with the presentinvention the redirection of one or more beams in a multi-beam system isaccomplished without the need for a spinning dove prism or wedge such asthose described in the above referenced related Ser. No. 08/687,931application.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a first embodiment of a multi-beam scanning system havinga two direction beam deflecting lens element in accordance with thepresent invention.

FIG. 2 depicts a second embodiment of a multi-beam scanning systemhaving a beam deflecting acoustic-optical modulator (AOM) element andpixel clock in accordance with the present invention.

FIG. 3A is a perspective view of the AOM, carriage assembly and drumdepicted in FIG. 2.

FIG. 3B shows the rotation of the AOM coordinate axes depicted in FIG.3A on the imaging surface.

FIG. 3C shows the correction of off-axis bow by the beam deflecting AOMdepicted in FIG. 2.

FIG. 4 depicts a third embodiment of a beam scanning system having a twodirection beam deflecting lens element and a deflected beam detector inaccordance with the present invention.

FIG. 5A depicts a forth embodiment of a beam scanning system similar tothat depicted in FIG. 4, but with a beam deflecting acoustic-opticalmodulator element and pixel clock substituted for the beam deflectinglens element of FIG. 4 in accordance with the present invention.

FIG. 5B depicts a slightly modified version of the beam scanning systemdepicted in FIG. 5A in accordance with the present invention.

FIG. 6 depicts a fifth embodiment of a beam scanning system having abeam deflecting lens element, a dual beam generation module (BGM) and adeflected beam detector in accordance with the present invention.

FIG. 7 details the BGM depicted in FIG. 6.

FIG. 8 depicts a sixth embodiment of a beam scanning system similar tothat depicted in FIG. 6, but with a beam deflecting acoustic-opticalmodulator element and pixel clock substituted for the beam deflectinglens element of FIG. 6 in accordance with the present invention.

FIG. 9 depicts a seventh embodiment of a multibeam scanning systemhaving a single direction beam deflecting lens element in conjunctionwith a beam deflecting acoustic-optical modulator element in accordancewith the present invention.

FIG. 10 depicts an eight embodiment of a multibeam scanning systemhaving a single direction beam deflecting lens element in conjunctionwith a pixel clock in accordance with the present invention.

FIG. 11 depicts a ninth embodiment of a multi-beam scanning systemhaving respective beam deflecting AOMs in conjunction with a pixel clockfor deflecting multiple beams in accordance with the present invention.

FIG. 12 details the BGM depicted in FIG. 11.

FIG. 13 depicts a tenth embodiment of a multi-beam scanning systemhaving two beam deflecting AOMs in a single beam path.

FIG. 14 details the BGM depicted in FIG. 13.

DESCRIPTION OF PREFERRED EMBODIMENT

Before beginning the detailed description of the embodiments shown inthe drawings, it should be noted that although each of the preferredembodiments is described in terms of a two-beam system, those skilled inthe art will recognize that the systems depicted are not limited to twobeams but could have any number of beams. In the case of those systemswhich include a beam deflecting AOM, additional beams could be added bysimply providing additional beam deflecting channels in a multichannelAOM or additional beam deflecting AOM's which are separately clocked andcontrolled to provide the necessary beam deflections. In the case ofsystems including a beam deflecting lens, an additional lens could beadded to deflect each added beam. Of course, beams which impinge on thespin deflector at the spin axis would not need to be deflected. Ifmultiple beams are to be deflected, it would appear beneficial for thebeams to be redirected so as to impinge upon the spin deflector inpaired locations which are an equal distance from the spin axis andopposite to each other. The offset from the spin axis is alsoadvantageously minimized to the extent possible in view of the intendedapplication.

FIG. 1 depicts a first embodiment of a multi-beam scanning system 100with a beam deflecting lens element in accordance with the presentinvention. As depicted, the system includes a first laser 105 whichgenerates a laser light beam directed on a first path to theacoustic-optic modulator (AOM) 115 which modulates the first beam beforeit passes through focusing lens 125 and collimating lens 130. The beamis directed substantially along the longitudinal axis of the cylindricaldrum 175 which is substantially coincident with the spin axis of spinmirror 155. After collimating, the beam is directed to the beam combiner195.

Also depicted is a second laser light generator 110 which generates alaser light beam and directs the beam on a second path throughacousto-optic modulator 120 which independently modulates the secondbeam. The beam is then reflected off stationary mirror 135 to beamdeflector lens 180 which is controlled by controller 185 to translate intwo directions simultaneously thereby redirecting the second beam inproper relationship with the rotation of the spin mirror. It should benoted that the above described AOM's serve solely as modulators andcould be eliminated if desired by performing modulation at each laser ina conventional manner using for example, clocked laser diodes.

The beam deflector lens 180 can be configured in an assembly and drivenas described in the above referenced related Ser. No. 08/687,928application. As described in detail therein, the beam deflector lens 180can be driven by electromagnetically, piezoelectrically or otherwise totranslate in at least two orthogonal directions about its stationaryoptical axis to deflect the beam emitted by laser 110 so as to impingeupon the spin mirror 155 at a desired point a distance from the spinaxis of the spin mirror 155. The two direction translation of the lensalso redirects the beam in synchrony with the rotation of the spinmirror 155 effectively maintaining the beam on the desired point on thespin mirror surface as the spin mirror rotates. This redirection of thebeam results in non-intersecting, e.g. parallel, scan lines with minimalbow, being produced by the first and second beams reflected off the spinmirror 155 onto the imaging surface 165. That is, the first beam isdirected at the point on the reflecting surface at the spin mirror whichintersects the spin axis. This beam is unaffected by the rotation of thespin mirror. The other beam is redirected such that it impinges on thereflecting surface of the spin mirror at a desired point on the mirrorsurface which is a preset distance from the spin axis. Accordingly, thelatter beam moves so as to continue to impinge upon the mirror at thedesired point as the spin mirror is rotated. In this way, both beamswill form parallel scan lines on the imaging surface.

The redirected beam propagates from the beam deflector lens 180 throughthe collimating lens 140 and to beam combiner 195. The combined beamsare then directed from the beam combiner 195 through collimating lens145 and focusing lens 150 so as to impinge upon spin mirror 155. Spinmirror 155 is driven by scan motor 160 to rotate, thereby scanning thelight beams onto the curved imaging surface 165 mounted insidecylindrical drum 175. As depicted, the focusing lens 150, spin mirror155 and scan motor 160 all form part of the carriage assembly 170 whichis moveable longitudinally through the drum 175. However the entiresystem, including the lasers 110 and 105, and controller 185, could ifdesired be included in the carriage assembly.

As described above, the system depicted in FIG. 1 provides dual scanningbeams on a curved imaging surface 165 without the need for a spinelement, such as a spinning dove prism or wedge, or a wobbling elementsuch as a piezo control wobble mirror, to provide non-intersectingmultiple scan lines on the curved imaging surface 165. Accordingly, asubstantially simplified system is provided in which only a singleprecessing or translating element is required to redirect a beam whichwill impinge upon a rotating spin deflector such that multiplenon-intersecting and preferably parallel beams will scan the curvedimaging surface disposed inside the cylindrical drum.

FIG. 2 depicts another system 200 for providing multiplenon-intersecting beams on a curved imaging surface without the need of aspin or wobble element. As depicted in FIG. 2, a laser 210 generates alaser light beam. The beam is directed to a beam splitter 215 which mayfor example be a multi-layer dielectric coated mirror or a gratingelement having a predefined wavelength. The two beams emitted from thebeam splitter 215 are focused by focusing lens 219 and directed througha dual channel AOM 220.

The first beam is directed through a first channel 220A of the dualchannel AOM 220 thereby generating a first propagating first order beamA. The second beam is passed through a second channel 220B of the dualchannel AOM 220 to generate a second propagating first order beam. Thedual channel AOM 220 is a beam deflecting AOM which is controlled bycontroller 218 to respectively deflect the first order beams in a singledirection with respect to the spin axis of the spin mirror 235, suchthat each forms a separate linear unfocused scan line across the spinmirror 235 and offset from the spin axis of spin mirror 235, whilesimultaneously modulating the beams. In this embodiment both laser beamsA and B are offset from the spin axis by an equal and opposite amount inthe axis orthogonal to the AOM deflection axis by a set of fixed angleoffsets in beam combiner 226. The unfocused beams A and B are redirectedto move linearly across the spin deflector 235 by an amount equal to onepixel in length at the imaging surface. The scan length is substantiallyless than the unfocused beam diameter at the spin mirror 235. The beamsare overlapping. The actual amount of overlap will depend on the spotsize, optical gain and other factors.

Pixel clocks 212A and 212B and clock phase shifters 214A and 214B arealso connected to the AOM 220 via video data registers 216A and 216B andAOM drivers 217A and 217B, which produce radio frequency (RF) sweepsignals modulated by the video data, to time emissions from therespective AOM channels 220A and 220B on a pixel by pixel basis suchthat the emission of the laser light which is directed through therespective channels 220A and 220B of the dual channel AOM 220 aresynchronized with the rotation of the spin mirror 235 and so as to phaseshift the beams passing through the respective channels of the dualchannel AOM 220. The respective timing of each beam modulation iscyclically adjusted synchronous with spinner rotation to compensate forthe in-scan component of bow error on the image surface and to eliminatethe in-scan component of the rotating vector representing the AOMdeflection axis projected onto the image surface thereby providing fordimensional correction of pixel placement error, i.e. scan line bow, atthe image surface. By applying the proper amounts of cyclic AOMdeflection and pixel clock phase shift in synchronism with the spinmirror 235, the two dimensional placement errors of the respective laserbeams are corrected without requiring the beams to be acted upon by anelement such as a spinning dove prism or wedge, a wobbling mirror, or atranslating lens. Accordingly, the respective laser beams are deflectedcyclically and simultaneously to form a linear scan on the spin mirror235 and the rotation of the spin mirror 235 scans the linearly scannedbeams on the imaging surface 255 as two non-intersecting scan lines withminimal bow.

If desired, a third beam could be directed through a non-deflecting AOMchannel and along a path coincident with the spin axis of the spinmirror 235, and preferably with the longitudinal axis of the cylindricaldrum 250. The beams emitted from the dual channel AOM 220 are focusedand collimated in the zero order mask and collimation optics 225 andcombined in beam combiner 226 into a pair of nearly coaxial beams havinga small offset angle relative to the spin axis of spin mirror 235. Thecombined beams are focused to two points on the imaging surface 255 byfocusing lens 230 before impinging on spin mirror 235. The spin mirror235 is rotated by spinner motor 240 such that two focused beams aredeflected onto the imaging surface 255 of the cylindrical drum 250. Asdepicted, the focus lens 230, the spin mirror 235, and spinner motor 240are all part of a carriage assembly 245 which moves longitudinallythrough the cylindrical drum 250.

The proper selection of the angles for the reflective surfaces of thebeam combiner 226 establishes the fixed beam offset angles in the beamseparation axis S, as shown in FIG. 3A, such that after opticalmagnification by beam expander optics 227, a one pixel separationbetween beams A and B at the imaging surface results. The depicted beamexpanding telescope expands the diameter of the collimated beams forprojection along the axis of the drum towards the linearly translatingcarriage assembly 245 to be intercepted by the final focus lens 230 andspin mirror 235. Alternatively, the optics preceding the beam combinermay be used to set the fixed offset angles. The beam combiner 226 canhave additional functions such as to split off a small amount of lightto a beam intensity sensor which provides closed loop feedback to theAOM drivers 217A and 217B to maintain constant beam intensity during thecyclic RF frequency sweep function.

It will be recognized by those practiced in the art that the functionsof the zero order mask and collimation optics 225, the beam combiner226, and the beam expander 227 can be replaced with a single telescopeand zero order mask without a beam combiner, in which case the physicalseparation distance between the channels in the dual channel AOM 220 isused to established the one pixel separation at the imaging surface.Various other means could also be used to provide a fixed one pixelseparation between beams A and B at the imaging surface.

FIG. 3A provides a perspective view of certain components depicted inFIG. 2. Like elements are identified with like reference numerals. Asshown, the channels of the dual channel AOM 220 have a channelseparation of D along the axis S which is reduced by optical means to asingle pixel separation D when projected on the imaging surface 255.Orthogonal to axis S is the AOM deflection axis F. FIG. 3B depicts theseaxes as projected on the imaging surface 255 of the cylindrical drum 250by the spin mirror 235 as it is rotated by the spinner motor 240. TheAOM deflection axis F rotates at the image surface 255 proportionatelywith the rotation angle of the spin mirror 235 as shown.

The uncorrected positions of the two laser beams A and B also rotatesynchronous with the spin mirror 235 but remain orthogonal to the AOMdeflection axis F. Also shown is the in-scan pixel timing axis T whichremains fixed and does not rotate with the spin mirror. The AOMdeflection axis F can be divided into an in-scan component Fx and across-scan component Fy. It should be noted that the AOM deflection axisF has a useful cross-scan component, Fy, at all spin mirror angles wherecross-scan correction is required.

Referring to FIG. 3C, the two scan lines designated A and B on theimaging surface 255 are depicted as the carriage assembly is movedlongitudinally through the drum 250. The solid lines indicate therespective scan line bow of the two off axis beams as they would appearwithout the compensation provided by each channel of the dual channelAOM 220 and pixel clocks 212A and 212B and phase shifters 214A and 214B,as described above. More particularly, as indicated by the solid lines,without the compensation the scan line separation in the cross-scandirection on the image surface would be zero when the spin mirror 235 isat ±90° rotation angle, increasing sinusoidally until a peak separationis reached at the 0° rotation angle of the spin mirror 235. The in-scandisplacement error component is also depicted in which scan line Aextends beyond scan line B by one pixel when the spinner is at ±90°rotation angle, with this phase error decreasing sinusoidally until itreaches zero at the 0° rotation angle. The dotted lines show thecorrected paths of beams A and B respectively.

To compensate for the cross scan displacement error at the imaging plane255 the AOM deflection axis F projected on the imaging plane 255 has auseful cross scan deflection component at all points, except at a 0°rotation angle of the spin mirror 235, where no cross scan compensationis required. However, there is also an unwanted in-scan component to thedeflection caused by each beam deflecting channel of the dual channelAOM 220. The pixel timing of the emitted scanning light beams set bypixel clocks 212A and B is advanced or delayed by the phase shifters214A and B, as appropriate, under the control of the controller 218, soas to cancel out the in-scan component of the deflection of each laserbeam in the beam deflecting channels of the dual channel AOM 220 and,additionally, to correct the uncompensated in-scan displacement errorcomponent shown in FIG. 3C. Accordingly the scan displacement errorsproduced by the laser beams impinging the spin mirror 235 at pointsother than coincident with the spin axis of the spin mirror 235 areeliminated by applying cyclical AOM deflection to the beam propagated bythe respective AOM channels, in synchrony with a cyclic phase controlsignal applied to the pixel clock signals for the beams directed to thechannels. It should be understood that advantageously each channel isseparately compensated, although this is not necessarily mandatory andthe system could be modified so that only one beam is off-axis such thatonly one phase shifter and one AOM deflection element could be used ifdesired.

FIG. 4 detects a still further embodiment of the present invention inwhich a beam deflector lens is utilized to move, e.g. rotate, a beam ina multi-beam system about the spin axis of a deflector element toprovide two non-intersecting beams on an imaging surface. The FIG. 4system 400 is similar to the previously proposed system described in theabove-mentioned '351 patent but provides substantially simplifiedoperation by eliminating the piezo controlled wobble mirror.

As shown in FIG. 4, a laser 426 directs a single beam to stationarymirror 428. The beam is reflected off stationary mirror 428. The laserpower is equally divided into S and P polarized beam components whichare separated by means of a polarization sensitive beam splitter 430with the respective beams directed from the beam splitter 430 alongseparate paths. The beam splitter 430 could be a multi-layer dielectriccoated mirror or a grating element.

The S beam is folded by the stationary mirror 432 and focused by lens434 prior to being directed through the acousto-optic modulator (AOM)438. From AOM 438 the beam is directed through collimating lens 442 tobeam combiner 446. The P beam is directed through focusing lens 436 andAOM 440 before being collimated by collimating lens 444. In thisembodiment, AOM's 438 and 440 serve solely as modulators, i.e. do notdeflect the P-beam or the S-beam. The collimated P beam is reflected bystationary mirror 460 along a path passing through the beam deflectorlens 462 which is a translating lens similar to beam deflector lens 180of FIG. 1. The lens is translated in at least two orthogonal directionssimultaneously to precess or rotate about its stationary optical axis,thereby deflecting the P polarized beam such that it rotates about thespin axis of the deflector (not shown), as has been previouslydescribed.

The beam combiner 446 is essentially identical to the beam splitter 430,but operated in reverse. The combined beam is transmitted to a quarterweight plate 448 and collimator optics 450. Collimator optics 450include a beam expanding lens 452 and a collimating lens 454. The axisof the collimator optics 450 is the axis along which the S polarizedbeam is projected. Preferably this axis is coincident with the spin axisof the spin deflector and the longitudinal axis of the drum.

The beam deflector lens 462 is under the control of control electronics424. The control electronics 424 control the driving of the lens 462 soas to translate in a precessing or rotational movement about itsstationary optical axis i.e., the axis of the lens 462 when in astationary position, to move the P polarized beam in synchrony with therotation of the spin deflector, as has been previously described.

The FIG. 4 embodiment further includes a quadrature photodetector 464for monitoring the relationship between the S polarized and P polarizedbeams downstream of beam deflector lens 462. Signals from the quadraturephotodetector 464 are processed by processor 422 which may operateeither in real time or non-real time to provide corresponding signals tothe control electronics 424 to control or modify the control of the beamdeflector lens 462. Accordingly, a small portion of both the P beam andthe S beam is transmitted from the beam combiner 446 onto quad detector464 via focusing lens 462.

The quarter wave plate 448 imposes a phase shift of 90° at thewavelength of the laser beams. Accordingly the combined beam whichimpinges upon the spin deflector (not shown) are dual beams withcircular polarization of opposite states. The feedback signals from thephotodetector 464 are used in conjunction with signals representing therotation of the spin deflector, such as signals issued by a deflectorshaft encoder, to monitor, control or adjust rotation of the P beamabout the S beam. The S polarized beam is directed coincident with thespin axis of the spin deflector. Accordingly, these signals are used tosynchronize P beam rotation with the angular position of the spindeflector, as will be well understood by those skilled in the art.

FIG. 5A depicts a multi-beam scanning system 500 which is similar tothat depicted in FIG. 4, but in which the acousto-optic modulator 440 isreplaced with a beam deflecting acousto-optic modulator 440', thecontrol electronics 424 is replaced by control electronics/phase shifter524, and a pixel clock 427 is provided. Also eliminated in the FIG. 5system is the beam deflector lens 462 of FIG. 4. As discussed above inconnection with the FIG. 2 system, the beam deflecting AOM 440', pixelclock 427 and control electronics/phase shifter 524 are conjunctivelyoperated with the spin deflector as previously described to redirect theP beam about the S beam. The S beam is preferably directed coincidentwith the spin axis of a spin deflector (not shown), and accordingly isunaffected by the rotation of the spin deflector.

FIG. 5B depicts a multi-beam scanning system 500'which is similar to themulti-beam system depicted in FIG. 5A. Accordingly, like elements aredesignated with identical reference numerals. In the FIG. 5B system thepixel clock 427' and control electronics/phase shifter 524' control theemission of the beam radiating from the laser source 426', in lieu ofcontrolling the emissions from the AOM 440". Accordingly, the AOM 440"is controlled by the control electronics/phase shifter 524' to deflectthe beam to move linearly and the laser 426' is controlled by the pixelclock 427' and control electronics/phase shifter 524' to phase shift thebeam thereby redirecting the P beam to scan the rotating spin deflectorin phase alignment with the S beam. An addition laser 426" is provided,thereby eliminating the need for beam splitter 430 of FIG. 5A. Also,stationary mirror 432' is substituted for mirror 432 due to the additionof laser 426".

FIG. 6 depicts still another embodiment of a multi-beam scanning system1300 in accordance with the present invention. As depicted a dual beamgeneration module (BGM) 1310 emits dual beams, one of which is directedthrough a beam deflector lens 1380 which is substantially similar to thebeam deflector lens described above with reference to FIG. 1. The secondbeam is directed to stationary mirror 1312 and reflected by stationarymirror 1312 to beam combiner 1314.

As shown in FIG. 7, the dual beam BGM 1310 includes a helium neon laser1405 which directs a beam of light through a beam splitter 1410. Fromthe beam splitter 1410, a first beam is propagated along the first pathto stationary mirror 1445 which reflects the beam through focusing lens1450. The focused beam is modulated in acousto-optic modulator (AOM)1455 and collimated by collimating lens 1460 before being emitted fromthe BGM 1310. A second beam is transmitted on a path through focusinglens 1415. The focused beam is modulated by AOM 1420 and collimated bycollimating lens 1425.

Returning to FIG. 6, the beam rotated by beam deflector lens 1380 andthe beam reflected from stationary mirror 1312 are combined in beamcombiner 1314. A portion of the combined beam is directed by beamsplitter 1345 along a path through focusing lens 1355. The focused beamis reflected by stationary mirror 1360 through a microscope objectivelens 1365 and onto a detector 1370 which may be a quadraturephotodetector similar to that described with reference to FIG. 4.Signals from the detector 1370 are processed in processor 1390 whichinstructs the controller 1395 to properly control the beam deflectorlens 1380 such that the beam passing through the deflector lens 1380 isproperly rotated about the beam reflected by stationary mirror 1312which is directed coincident to the spin axis of the spin mirror 1330.

The combined beams are directed to the collimating lens 1350 and focusedby focusing lens 1325 onto the imaging surface 1335. The spin mirror1330 deflects the combined beams to form 2 non-intersecting scan lineson the imaging surface 1335 of the cylindrical drum 1340. As depicted inFIG. 6 the focusing lens and spin mirror are a part of a carriageassembly 1320 which is configured to move longitudinally through thedrum 1340. As in the previous embodiments, the spin axis of the spinmirror 1330 is preferably coincident with the longitudinal axis of thecylindrical drum 1340 and the beam reflected by mirror 1312 ispreferably directed coincident with the spin axis of the spin mirror1330.

FIG. 8 depicts a still further embodiment of a multi-beam system 1300'in accordance with the present invention. This embodiment is similar tothe FIG. 6 embodiment and like components are identified with likereference numerals. However, in this embodiment the beam deflector lens1380 of the FIG. 6 embodiment has been replaced by a beam deflecting AOM1455', the controller 1395 is replaced by controller/phase shifter 1395'and a pixel clock 1512 is added. The AOM 1455' is clocked and controlledby the controller 1395' as previously discussed in detail. The AOM 1455'is thereby controlled so as to deflect and phase shift one of the beamsemitted from the dual beam BGM 1310' so that the beam is redirected toform a scan line across the spin mirror 1330 and be in phase with theother beam emitted by the dual beam BGM 1310'. The light beam isredirected such that the beams deflected by spin mirror 1330 form twonon-intersecting e.g. parallel, scan lines, with minimal bow, on theimaging surface 1335 as has been described in detail above.

FIG. 9 depicts a still further embodiment of the subject invention whichcombines aspects of the FIGS. 1 and 2 embodiments to redirect at leastone beam in a multi-beam scanning system to scan a curved imagingsurface with multiple non-intersecting scan lines. The FIG. 9 system100' is similar to FIG. 1 except as will be discussed below.Accordingly, like components are referenced with like referencenumerals.

As depicted in FIG. 9, a laser 105 emits a first light beam directed ona path coincident with the spin axis of the spin mirror 155 just as inthe FIG. 1 embodiment. A second laser 110 emits a beam which is receivedby a beam deflecting acousto-optic modulator 120' which deflects theemitted beam in a first direction. The AOM 120' is substantially similarto one channel of the dual channel AOM 220 described above withreference to the FIG. 2 system.

The deflected beam is then reflected off stationary mirror 135 along apath through a beam deflector lens 180' which is configured to translatein a single direction to deflect the beam in a direction orthogonal tothe direction in which the AOM 120' deflects the beam emitted by laser110. The controller 185' conjunctively controls the operation of AOM120' and beam deflector lens 180' to cause the beam emitted from thedeflector lens 180' to be redirected to move about an axis coincident tothe path of the beam emitted by laser 105, and hence with thelongitudinal axis of the drum 175 and the spin axis of the spin mirror155. Accordingly, like the other embodiments of the present invention,multiple non-intersecting scan lines can thereby be formed on the curvedimaging surface 165 of the cylindrical drum 175.

Turning now to FIG. 10, another hybrid embodiment of the presentinvention is shown. FIG. 10 is also similar to FIG. 1 and likecomponents are identified with like reference numerals. As shown, in thesystem 100" the second light beam emitted by laser 110 is again directedthrough the acousto-optic modulator 120 and on to the stationary mirror135. The beam deflector lens 180' is similar to beam deflector lens 180'of FIG. 9 and is configured to translate in a single direction todeflect the beam to linearly scan on spin mirror 155.

A pixel clock 112' and controller/phase shifter 185" are provided tocontrol the timing of emissions from AOM 120, or alternatively ifdesired from the laser 110', in a manner similar to that of pixel clock427' and phase shifter 524' described in connection with the FIG. 5Bembodiment. Accordingly, by providing the proper timing signals to theAOM 120 the laser light beam emitted by AOM 120 is phase shifted so asto be phase aligned with the other beam when scanned on imaging surface165.

Both the pixel clock 112' signal phasing and beam deflector lens 180'are controlled by controller/phase shifter 185" to conjunctively operatein synchrony such that the beam emitted from the beam deflector lens180' is redirected to move cyclically or oscillate and thereby form asingle scan line on the spin mirror 155. Accordingly, non-intersecting,phase aligned multiple scan lines will be formed on the curved imagingsurface 165 of the drum 175.

FIG. 11 depicts a ninth embodiment of a multi-beam scanning system1300". The system is similar to that depicted in FIGS. 6 and 8 andaccordingly like elements are identified with like reference numerals.The scanning system 1300", however, does not include a beam deflectinglens or a beam deflecting AOM which is external to the beam generationmodule. As indicated in FIG. 12 which details the BGM 1310", the AOMs1420" and 1455" are each controlled by the controller/phase shifter1395" and are associated with pixel clocks 1512A" and 1512B",respectively, which are operated such that the AOMS 1420" and 1455"deflect and phase shift the respective beams emitted from the BGM 1310"so as to be phase aligned and redirected to linearly scan the spinmirror 1330" and in synchrony with the rotation thereof. It should benoted that AOM 1420" and AOM 1455" simultaneously modulate and deflectthe beams. Further, preferably the beams are deflected so as to impingeupon the spin mirror 1330" at an equal distance from the spin axis andat opposed points on the mirror 1330".

Turning now to FIGS. 13 and 14. A tenth embodiment of a multi-beamscanning system 1300'" is depicted. System 1300'" is similar to thesystems depicted in FIG. 6 and particularly FIG. 8. In system 1300'",two AOMs 1455'" and 1575' are utilized to deflect one of the beams intwo directions to cause the beam to be redirected to move about the spinaxis of the spin mirror 1330 and in synchrony therewith. Both AOMs1455'" and 1575' are controlled by the controller 1395'" in the samemanner as has been discussed above, the respective AOMs being controlledto deflect each the beam in different orthogonal directions.

As described above, the present invention provides a multi-beam scanningsystem which does not require a spin or wobble element to rotate one ormore of the multiple beams. The described scanning systems has reducedbanding and/or twinning. The scanning systems are capable of scanningmultiple beams having a desired geometric relationship with respect toeach other. The scanning systems also can scan multiple beams in adesired manner with respect to the spin axis of a spin deflector.

It will also be recognized by those skilled in the art that, while theinvention has been described above in terms of preferred embodiments itis not limited thereto. Various features and aspects of the abovedescribed invention may be used individually or jointly. Further,although the invention has been described in the context of itsimplementation in a particular environment and for particularapplications, e.g. electronic prepress applications, those skilled inthe art will recognize that its usefulness is not limited thereto andthat the present invention can be beneficially utilized in any number ofenvironments and implementations. Accordingly, the claims set forthbelow should be construed in view of the full breadth and spirit of theinvention as disclosed herein.

We claim:
 1. A multibeam scanning system for scanning a curved imagingsurface, comprising:at least one radiation emitter configured to emit afirst beam of radiation and a second beam of radiation; a spindeflector, rotatable about a spin axis, configured to direct the firstbeam to form a first scan and the second beam to form a second scan lineon said imaging surface; and at least one translating element, disposedin the path of at least one of the first and the second beams andupstream of said spin deflector, translatable in at least twosubstantially orthogonal directions, and operable to deflect said atleast one beam with respect to the spin axis of the spin deflector.
 2. Ascanning system according to claim 1, wherein said at least onetranslating element is configured to deflect the at least one beam suchthat the first scan line and the second scan line are non-intersecting.3. A scanning system according to claim 1, wherein said at least onetranslating element is translatable in synchrony with rotation of saidspin deflector.
 4. A scanning system according to claim 1, furthercomprising a detector configured to detect a geometric relationshipbetween the second beam and the first beam.
 5. A scanning systemaccording to claim 4, further comprising a controller and wherein saidcontroller is configured to control translation of said at least onetranslating element in accordance with the detected relationship.
 6. Ascanning system according to claim 4, wherein said detector includes aquadrature photodetector.
 7. A scanning system according to claim 4,wherein the detector is configured to detect the geometric relationshipprior to writing on the imaging surface.
 8. A scanning system accordingto claim 1, wherein said first beam impinges upon the spin deflector ata first location relative to the spin axis of the spin deflector and theat least one translating element is configured to deflect the secondbeam so as to impinge upon the spin deflector at a second location whichis different from the first location.
 9. A scanning system according toclaim 8, wherein said first location is on the spin axis of the spindeflector.
 10. A scanning system according to claim 1, wherein the atleast one translating element is a beam deflecting lens.
 11. A scanningsystem according to claim 1, further comprising a pixel clock configuredto issue a signal for timing the emitting of radiation from one of saidat least one radiation emitter, and a phase shifter for adjusting thetiming of the signal, wherein:the at least one translating elementincludes a beam deflecting lens to deflect the second beam to movelinearly across the spin deflector; the pixel clock and the phaseshifter are operable in synchrony with the operation of the beamdeflecting lens to phase shift the second beam; and the deflection ofthe second beam in relationship with rotation of the spin deflectorabout the spin axis and the phase shifting of the second beam result inthe second scan line being non-intersecting and in phase with the firstscan line.
 12. A scanning system according to claim 1, furthercomprising a beam deflecting acousto-optic modulator configured todeflect said second beam, wherein:the at least one translating elementincludes a beam deflecting lens operable to deflect the second beam in afirst direction and in a second direction; and the deflection of thesecond beam in the first and the second directions causes the secondbeam to move in synchrony with rotation of the spin deflector about thespin axis, such that the first scan line and the second scan line arenon-intersecting.
 13. A method of scanning a curved imaging surface withmultiple beams, comprising:emitting a first beam of radiation and asecond beam of radiation; rotating a spin deflector about a spin axis todirect the first beam to form a first scan line and the second beam toform a second scan line on said imaging surface; and simultaneouslytranslating an element in two substantially orthogonal directions, theelement being disposed in the path of the second beam and upstream ofsaid spin deflector, to deflect said second beam with respect to thespin axis of the spin deflector.
 14. A scanning method according toclaim 13, wherein said translating step includes translating the elementto deflect the second beam such that the first scan line and the secondscan line are non-intersecting.
 15. A scanning method according to claim13, wherein said translating step includes translating the element insynchrony with rotation of said spin deflector.
 16. A scanning methodaccording to claim 13, further comprising the step of detecting ageometric relationship between the second beam and the first beam afterdeflection of the second beam.
 17. A scanning method according to claim16, further comprising the step of controlling the translating of theelement in accordance with the detected relationship.
 18. A scanningmethod according to claim 16, wherein the step of detecting is performedprior to writing on the imaging surface.
 19. A scanning method accordingto claim 13, wherein the first beam impinges upon the spin deflector ata first location relative to the spin axis of the spin deflector and thestep of translating includes deflecting the second beam so as to impingeupon the spin deflector at a second location which is different from thefirst location.
 20. A scanning method according to claim 19, whereinsaid first location is on the spin axis of the spin deflector.
 21. Ascanning method according to claim 13, wherein the translating stepincludes translating the element in multiple directions.
 22. A scanningmethod according to claim 13, further comprising the step of timing theemitting of radiation and wherein:the translating step includestranslating the element to deflect the second beam to form a linear scanacross the spin deflector; the emitting of radiation is timed so as tobe synchronized with the deflection of the second beam to thereby phaseshift the second beam; and the linear scanning of the second beam isperformed in relationship with rotation of the spin deflector about thespin axis such that non-intersecting scan lines are formed on theimaging surface, and the phase shifting causes the non-intersecting scanlines to be in phase.
 23. A scanning method according to claim 13,wherein opticallythe deflection of the second beam in the first and thesecond directions causes the second beam to move in synchrony withrotation of the spin deflector about spin axis such that the first scanline and the second scan line are non-intersecting.
 24. A multibeamscanning system for scanning a curved imaging surface, comprising:acylindrical drum having a curved imaging surface disposed therein; atleast one radiation emitter configured to emit a first beam of radiationand a second beam of radiation; a spin deflector, configured to rotateabout a spin axis and thereby direct the first beam to form a first scanline and the second beam to form a second scan line on said curvedimaging surface; and a translatable lens operable to deflect said secondbeam with respect to the spin axis to cause the second beam to move inrelationship with the rotation of the spin deflector about the spin axissuch that the first scan line and the second scan line arenon-intersecting; wherein said translatable lens is disposed in only apath of the second beam and upstream of said spin deflector.
 25. Ascanning system according to claim 24, further comprising a pixel clockconfigured to issue a timing signal for timing emissions from said atleast one radiation emitter and a phase shifter for adjusting the timingof the signal, wherein:the translatable lens is configured to translateand thereby deflect said second beam to linearly scan the spindeflector; the pixel clock and phase shifter are operable in synchronywith the translation of the translatable lens to phase shift the secondbeam; and the deflection of the second beam in relationship withrotation of spin deflector about the spin axis and the phase shifting ofthe second beam cause the scan lines to be non-intersecting and inphase.
 26. A scanning system according to claim 24, further comprising abeam deflecting acousto-optic modulator operable to deflect the secondbeam, wherein:the translatable lens is configured to translate in asingle direction to deflect the second beam in a first direction; theacousto-optic modulator is adapted to operate in synchrony with thetranslation of the translatable lens to deflect the second beam in asecond direction, substantially orthogonal to the first direction; andthe deflection of the second beam in the first and the second directionscauses the second beam to move around the spin axis.
 27. A multibeamscanning system for scanning an imaging surface, comprising:at least oneradiation emitter configured to emit a first beam of radiation and asecond beam of radiation; a spin deflector configured to rotate about aspin axis and thereby direct the first beam to form a first scan lineand the second beam to form a second scan line on said imaging surface;at least one moving element operable to deflect at least one of saidfirst and said second beams in relationship with the rotation of thespin deflector about the spin axis to cause said deflected at least onebeam to form a linear scan on the spin deflector; at least one pixelclock configured to issue a timing signal for timing emissions ofradiation from the at least one radiation emitter; and at least onephase shifter configured to adjust timing of the signal; wherein the atleast one pixel clock and the at least one phase shifter are operable insynchrony with movement of the moving element to phase shift at leastone of said first and said second beams such that the scan lines arenon-intersecting and path aligned.
 28. A scanning system according toclaim 27, further comprising a cylindrical drum having a curved imagingsurface and wherein the moving element is a translatable lens configuredto translate in only a single direction.
 29. A multibeam scanning systemfor scanning a curved imaging surface, comprising:at least one radiationemitter configured to emit a first beam of radiation and a second beamof radiation; a spin deflector, rotatable upon a spin axis, configuredto direct the first beam to form a first scan line and the second beamto form a second scan line on said imaging surface; at least onenon-reflecting element disposed in the path of the second beam andupstream of said spin deflector, operable to deflect said second beamwith respect to the spin axis of the spin deflector; and a detectorconfigured to detect a geometric relationship between the second beamand the first beam.
 30. A system according to claim 29, wherein the atleast one non-reflecting element is a lens translatable in a firstdirection and a second direction, the first and second directions beingseperated by an angle of other than 180 degrees.
 31. A system accordingto claim 29, wherein the at least one non-reflecting element includes atranslatable lens operable to deflect the second beam in a firstdirection and an acousto-optical element operable to deflect the secondbeam in a second direction different from the first direction.
 32. Amethod of scanning a curved imaging surface with multiple beamscomprising:emitting a first beam of radiation and a second beam ofradiation; rotating a spin deflector about a spin axis to direct thefirst beam to form a first scan line and the second beam to form asecond scan line on said imaging surface; operating at least onenon-reflecting element, disposed in the path of the second beam andupstream of said spin deflector, to deflect said second beam withrespect to the spin axis of the spin deflector; and detecting ageometric relationship between the second beam and the first beam.
 33. Amethod according to claim 32, wherein the at least one non-reflectingelement is a lens translatable in a first direction and a seconddirection, the first and second directions being seperated by an angleof other than 180 degrees.
 34. A method according to claim 32, whereinthe at least one non-reflecting element includes a translatable lensoperable to deflect the second beam in a first direction and anacousto-optical element operable to deflect the second beam in a seconddirection different from the first direction.